1. Field of the Invention
This invention pertains to the field of image analysis. More particularly, it pertains to image transformation or pre-recognition processing using adaptive quantization based on a local average.
2. Description of the Prior Art
The digital representation of an image by pixel values dispersed in a matrix is, well known, as are various digital techniques for the removal of noise from the image, "noise" being used in the sense of anything which degrades an ideal image and including high frequency noise, which affects one or a few pixels, and low frequency noise, such as a systemic error which causes a brightness change between opposite sides of an image.
An effective way of correcting an unrepresentative value of one pixel is "median filtration", which is the substitution, for the value of one pixel, of the median of pixel values in a predetermined neighborhood of pixels containing the one pixel. For example, the median value of pixels arranged in a cross or a square centered on the one pixel may be substituted for the value thereof. Such neighborhood median filtration is, however, relatively ineffective for low frequency noise and does not take advantage of lengthy discontinuities, such as the horizon or extended object portions, actually present in an imaged scene.